HR: 17:15h
AN: V44C-06    [Abstracts]
TI: Tracking the Longevity and Evolution of the Youngest Toba Tuff Using Allanite and Zircon Chronology, Thermometry, and Chemostratigraphy
AU: * Reid, M R
EM: mary.reid@nau.edu
AF: Northern Arizona University, Department of Geology, Flagstaff, AZ 86011-4099, United States
AU: Vazquez, J A
EM: jorge.vazquez@csun.edu
AF: California State University Northridge, Department of Geological Sciences, Northridge, CA 91330-8266, United States
AB: Accessory phases are powerful repositories of information about the thermal and chemical evolution of silicic magma bodies. Allanite and zircon from the relatively youthful and chemically variable Youngest Toba Tuff (YTT) of Indonesia provide key opportunities to explore gestation of Earth's largest Quaternary eruption. The YTT is compositionally zoned from 63 to 77 wt.% SiO₂ but high silica rhyolite is predominant. Chemical diversity within the YTT can be explained largely by crystal fractionation. We have previously determined that allanites entrained by the eruption began to crystallize as much as 150 k.y. before eruption of the YTT at ca. 73 ka, but most crystallized within 50 k.y. of eruption. Age ranges within individual allanites can be as large as 150 k.y. Allanites from felsic enclaves yield ages that are within tens of thousands of years before eruption, but are distinct in composition from allanites in their host pumice. Zircons, on the other hand, range in age to more than 500 k.y. before eruption, although most are within 300 k.y. of it. Rare xenocrysts of Paleocene (ca. 60 Ma) and late Proterozoic (ca. 700 Ma) age and probable antecrysts from the earlier Oldest Toba Tuff are also present. Individual zircons crystallized over ca. 30 to more than 500 k.y. intervals. Taken at face value, the wider age distribution of zircon compared to allanite could be explained by secular cooling of the magma reservoir, with zircon saturation occurring much earlier than allanite saturation. Zircon saturation temperatures, FeTi-oxides, and allanite compositions collectively suggest that most of the YTT crystallized and evolved over the temperature interval 700-800°C. Temperatures based on Ti-in-zircon geothermometry expand this range to 670- 830°C. The zircons thus appear to retain a more complete record of differentiation than the host lavas and may represent crystals retained from more mafic progenitors of and/or intrusions into the YTT as well as zircons remobilized from a (semi-)solid state. Zircon rims are, on average, more than 50°C lower than their cores and thus do not suggest mush remobilization by mafic intrusion. Some features of the chemical compositions of the zircons (e.g., Hf) correlate with the variation in temperature but neither the distribution of temperatures nor of chemical compositions are unique to pumice composition. Many cores are chemically distinct and even rim Y contents scatter despite expected bulk D(Y)~1. When combined with chronological information, there are no obvious secular chemical changes in zircons. The age, temperature, and chemical variations corroborate evidence from allanite for protracted crystallization within different, yet coeval, reservoir domains that differed in temperature and melt composition.
DE: 1036 Magma chamber processes (3618)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 1120 Isotopic disequilibrium dating
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting